I appreciate you bringing this to our attention, @anthonywx.
I'd like to step in and ensure that you can send an invoice within QuickBooks mobile app.
First, did you receive an error code when attempting to send an invoice?
I recommend clearing your QuickBooks Online (QBO) Mobile app's data. Cached data can cause difficulties while some tasks. You can accomplish this by following the steps highlighted below:
Go to the Menu ☰
Select Help & Feedback
Click Refresh Data
Then select Refresh.
Furthermore, we can uninstalling and reinstalling the app to ensure we are using the most recent version to avoid getting errors when sending invoices.
Here's how:
Tap and hold the app on your phone.
Drag the app to the Uninstall section.
Select Yes to confirm.
You can go to the following links to reinstall the QBO mobile app: